1. Field of Use
These teachings relate generally to forensic photograph dating and more particularly to systems employing digital computers for determining the probabilistic date of a physical characteristic associated with a photograph. The teachings may also relate to any field where reflectance transformation imaging is employed, such as, for example, the fields of ballistics investigation or numismatics.
2. Description of Prior Art (Background)
A photographic print's date is elementary to the understanding of the work, its historical context and the photographer's artistic intent. It carries implications for its treatment, display and storage and can manifestly influence its market value. Recently, photographs have become the target of forgers, and as the market value of these works increase, so will forgery continue. The detection of forged photography is particularly difficult in the context of today's imaging technology as experts must be able to tell the difference between originals and reprints. For example, a forger in possession of photo-negatives would allow the forger to print an unlimited number of prints, which then can be passed off as original.
Texture is a defining attribute of photographic paper. Starting in the early 20th century, manufacturers manipulated texture to differentiate their products and to satisfy the aesthetic and functional requirements of photographers. Prior to WWII, when black and white silver gelatin paper was the dominant photographic medium, dozens of manufacturers worldwide produced a wide array of surfaces. From this period a book of specimen prints by the Belgian company Gevaert lists twenty five different surfaces comprising combinations of texture, reflectance, color and paper thickness (Gevaert Company of America c. 1935). Around the same time, a sample book from the Defender Company of Rochester N.Y. lists twenty seven surfaces (Defender Photo Supply Company c. 1935), Mimosa twenty six (Mimosa AG c. 1935) and Kodak twenty two (Eastman Kodak c. 1935). Each listed surface was proprietary to the different manufacturers and each was used across their multiple brands of paper with changes, additions, and deletions occurring over a span of many years.
Texture, a vital factor in the evaluation of paper surface, impacts the visibility of fine detail and thus provides insight into the artistic intent of the photographer and the envisioned purpose of a particular print. For example, prints intended for reproduction or documentary functions tend to be better suited on smooth-surface papers that render details with sharpness and clarity; on the other hand, more impressionistic or expressive subjects, especially those depicting large unmodulated masses of shadows or highlights, are best suited for papers with rough, broadly open textures (Eastman Kodak Company c. 1935).
A result of a careful and deliberate manufacturing process, texture applied to silver gelatin paper is designed to be distinct and distinguishable through processing and post-processing procedures. Given these texture attributes, an encyclopedic collection of surface textures can reveal vital clues about a photographic print of unknown origin. Likewise a method for classifying textures can provide a means to link prints to specific photographers or to other prints of known provenance.
Since the composition of photographic paper was frequently changed, fake photographs are likely to be printed on modern photographic paper or photographic paper not contemporaneous with the original photograph. Therefore, there is a need for a system to non-destructively date photographic paper.
Determining photographic paper surface texture, a critical feature in the manufacture, marketing and use of photographic paper, is one way to non-destructively date photographic paper. Using a raking light can reveal texture through a stark rendering of highlights and shadows. Though raking light photomicrographs effectively document surface features of photographic paper, the sheer number and diversity of textures used for historic papers prohibits efficient visual classification.
In addition, the raking light may be applied to a sample paper with different angles of incidence and different intensities, thereby rendering different highlights and shadows for the same photograph or sample. Therefore, a need exists for a method and apparatus for standardizing and classifying photograph textures revealed by a raking light.